The diagnosis of a great number of contagious diseases is based on the evalution of the immunological response generated by the hosts against the invading infectious agents.
The methodologies for these types of studies are plentiful but nearly all of them are based on a similar principle; this principle consists in the quantification or semiquantification of the antibodies generated against the microorganisms causing the disease. This type of investigation falls within the specialty termed serology, and it is an indirect way of inferring the existence of a disease. In fact, it is argued that if a person possesses antibodies against a certain infectious agent, this is due to the fact that this agent or components of same (antigens) are causing an immunological stimulation of the host, and consequently the generation of antibodies could be assumed to mean that the infectious agent is present.
It is thus obvious that the efficiency of the system will depend on how reliably we can assign to each detection of antibodies its true correspondence with a certain antigen or microorganism. It is also obvious that the efficiency of the system will depend on the levels of detection of antibodies that can be achieved.
These two aspects are generally known as the specificity and sensitivity of serological tests. Of the two, sensitivity is perhaps easier to achieve as far as instruments are concerned, because due to the evolution and improvement of methods for measuring biological molecules, including radioisotopes, enzymes, etc., it is possible to detect infinitesimal quantities of any immunologically interesting protein.
Specificity is more difficult to deal with. Implicit in this question are problems derived from the antigenic stimulus and problems derived from the very responses whether natural or triggered by the stimulated host.
A great number of antigens are widely distributed in nature and repeated among living creatures. It is quite common, accordingly, that the response against one of them is capable of reacting simultaneously against several owners of the same.
In the course of the past 30 or 40 years, scientists doing research work in the field of immunochemistry have attached special importance to the search for antigens having a unique identity for the preparation of serological reagents. In some cases, these efforts have met with success. Nevertheless, they are not easy to accomplish and very often they are followed by a substantial reduction of the sensitivity inherent in the system.
It is obvious that the search for specific components of a microbial agent should be accompanied by a selection in which some components are dismissed and others favoured. The hosts do not react always with the same efficiency or selectivity to all the antigens comprising an infectious agent and accordingly the selection of some of them with detriment to others makes the analysis more specific, but at the expense of a reduced sensitivity, inasmuch as, regardless of the sophistication of the instruments, it is not possible to detect non existent elements in our composition.
In the serological research of some diseases, attempts have been made to solve problems of this type by "neutralizing" or "absorbing" similar but not specific immunological responses against the infectious agent under consideration. These neutralizations or absorptions were carried out with microorganisms similar to but not identical with those investigated. Progress was made in the case of the serological diagnosis of Syphilis by means of Indirect Immunofluorescence and in the case of the Chagas disease for reactivity crossed with Leishmaniasis. However, this method did not spread to other pathologies, whether due to lack of success in the results or due to a marked drop in the serological titers, or of specific responses, after the absorptions.
A different alternative to improve specificity was to work on the mechanisms of the natural or triggered response of the hosts. Several years ago, it was discovered that the immunoglobulins of the IgM type present a more nonspecific behaviour than those termed IgG. Immunoglobulins M are high molecular weight proteins, acting as an immunological ancestor: they are the first to appear in a foetus, the first to appear in an infection and, moreover, they comprise the best part of "natural antibodies". They are found to be active in the polymeric state, and if this is destroyed, the IgM loses nearly all its functions. The IgG molecules are low weight proteins and represent the most perfect and tardiest response of the host. They are active in monomeric state and this allows an easy differentiation from IgM. By means of a gentle reduction method which breaks the disulphide bonds that keep intact the polymeric state of the IgM, this molecule can be deactivated without affecting the IgG. This mechanism has brought about important improvements in the specificity of different serological tests; particularly of agglutination tests of cells or microorganisms. Such is the case of the Direct Agglutination of Chagas and Toxoplasmosis where the present inventor other authors as well, have been able to prove that the systematic use of 2-mercaptoethanol (a gentle reducing agent) allows with acceptable efficiency, presumably normal persons from others who have become infected.